Desirable clinical responses of man to treatment with thymic hormones in selected immunological disorders and certain types of cancer has created the need to understand better the immunoregulatory hormones in man and experimental animal models. Whereas the mouse model is valuable, other models, in particular the chicken because of its unique anatomic diversity of lymphoid organs, should be explored. A low molecular weight polypeptide which shares a number of physiochemical properties with bovine thymosin has been isolated from chicken thymus and characterized in our laboratory. In addition, the avian thymic hormone appears to be localized in the thymic reticuloepithelial cells, thus paralleling the observation with bovine thymosin. The aim of this proposal is to study the immunomodulating protential of the avian thymic hormone in vitro and in vivo using a variety of techniques. Bone marrow cell suspension which contain precursors of lymphoid cells will be fractionated on a BSA gradient. The different fractions will be incubated with varying concentrations of the hormone and assayed for rosette formation, mixed lymphocyte reaction, graft-vs-host reaction, lymphocyte blast transformation, cytotoxicity assay and induction of the expression of T lymphocyte surface markers and terminal deoxynucleotidyl transferase enzyme. The in vivo studies will evaluate the effect of treatment with the avian thymic hormone on naturally occuring or experimentally induced immunodeficiency disorders. The experimentally induced conditions will include chemical or surgical bursectomy alone or in combination with surgical thymectomy. These immunologically deficient birds will be reconstituted with various subpopulations of lymphocytes obtained using a cell sorter, followed by treatment with the avian thymic hormone and the resulting immunomodulation will be recorded and analyzed.